1. Field of the Invention
This invention relates to a reverberation-imparting device which imparts delay effects such as reverberation to an input signal thereto.
2. Prior art
Conventionally, there have been proposed so-called reverberation-imparting devices which impart delay effects such as reverberation to input signals thereto in order to simulate various sound field spaces.
As a reverberation-imparting device of this kind, a reflective reverberation-imparting device has been proposed by the present assignee in Japanese Patent Publication (Kokoku) No. 1-57799, which is capable of generating a reverberation sound close to natural sound as well as varying its reverberation characteristics with ease.
FIG. 1 shows, by way of example, waveforms of a reflected sound signal created by the proposed reflective reverberation-imparting device. This device creates an initial reflected sound waveform F simulating a reflected sound obtained by an initial reflection of a sound by a wall or the like, and a subsequent reflected sound waveform (reverberation sound waveform) S simulating reflected sounds obtained by second and subsequent reflections of the sound by the wall or the like. The reflected sound obtained by the initial reflection has relatively small changes in frequency components and amplitude and is merely delayed as compared with the original sound, and therefore can be simulated by the initial reflected sound waveform F shown in FIG. 1. On the other hand, the reflected sounds obtained by the second and subsequent reflections have relatively large changes in frequency components and amplitude as compared with the original sound. Further, these reflected sounds obtained by the second and subsequent reflections have various delay amounts which are individually different, because of the repeated reflections before the original sound is attenuated. Moreover, each reflected sound obtained by each reflection has reduced high frequency components and an increased amount of reduction in amplitude as compared with its immediately preceding reflected sound. Therefore, these subsequent reflected sounds can be simulated by the subsequent reflected sound or reverberation sound waveform S shown in FIG. 1, which is formed by a plurality of waveforms of reflected sounds overlapping one upon another, i.e. highly dense, and progressively declining.
FIG. 2 schematically shows the arrangement of the proposed reflective reverberation-imparting device. Left and right signals (hereinafter referred as "the signal L" and "the signal R", respectively) of a 2-channel stereo signal are added together by an adder 101, and the resulting sum is delivered to an initial reflected sound waveform-forming section 102 which forms the above-mentioned initial reflected sound waveform F. The initial reflected sound waveform-forming section 102 generates three kinds of signals which are different in delay time, etc. from each other, one of which is Supplied to a subsequent reflected sound or reverberation waveform-forming section (hereinafter referred to as "the reverberation waveform-forming section) 103 which forms the above-mentioned subsequent reflected sound or reverberation sound waveform S, and the other two signals are supplied to respective adders 104 and 105. The reverberation waveform-forming section 103 operates in response to the signal from the initial reflected sound waveform-forming section 102 to form signals L' and R' for forming the subsequent reflected sound or reverberation sound waveform S. The formed signals L', R' are added to the respective two signals from the initial reflected sound waveform-forming section 102 at the adders 104, 105, and the resulting sums are added to the respective signals L, R before being subjected to the addition by the adder 101, at respective adders 106 and 107, so that the reflected sound or reverberation sound waveform S shown in FIG. 1 is obtained. In FIG. 2, each symbol ".DELTA." represents a multiplier for multiplying an input signal thereto by a predetermined coefficient.
The initial reflected sound waveform-forming section 102 is mainly comprised of a delay line 111 formed by a RAM, and a pair of adders 112 and 113. Results of the addition from the adder 101 are successively written into the delay line 111 at predetermined time intervals so that signals are stored into locations A1 to A6 and A7 to A12 which are arranged at intervals corresponding to respective different predetermined delay times. The signals read from the locations A1 to A6 and A7 to A12 are delivered to the adders 112 and 113. Results of additions from the adders 112, 113 are the above-mentioned two signals supplied to the adders 104, 105 for forming the initial reflected sound waveform F in FIG. 1. The delay line 111 successively generates signals, which correspond to respective signals from the adder 101 written into the delay line 111 and are each delayed by a predetermined delay time, through a reading output (reading location) B, in the order of writing into the delay line 111. The successive signals from the reading output B are delivered to the reverberation waveform-forming section 3.
The reverberation waveform-forming section 3 is comprised of a plurality of comb filters 125 each formed of a delay line 121, a low-pass filter (hereinafter referred to as "the LPF") 122 for filtering out or removing high frequency components in an output signal from the delay line 121 through a reading output (reading location) D thereof, a multiplier 123 for attenuating an output signal from the LPF 122, and an adder 124 for adding together the aforementioned signal from the delay line 111 and an output signal from the multiplier 123, adders 126 and 127, the adder 126 for adding together left-channel signals from some of the comb filters 125 and the adder 127 right-channel signals from the other comb filters 125, and two pairs of all-pass filters (hereinafter referred to as "the APF's") 128, 129; 130, 131, the APF's of each pair being serially connected to each other as well as to the respective adder 126, 127, for changing the phases of respective output signals from the adders 126, 127, delaying the same, etc. The APF's 128-131 are identical in structure with each other.
The delay lines 121 of the comb filters 125 each have reading outputs (reading locations) C1 and C2 thereof located relative to the other delay lines 121 such that different amounts of delay occur between the comb filters 125 to thereby ensure that output waveforms formed thereby will have high density enough to form the subsequent reflected sound or reverberation sound waveform S. The LPF's 122 of the comb filters 125 remove high frequency components in the feedback output from the delay lines 121, as stated above, in order to simulate a sound repeatedly reflected from a wall or the like, because as the number of times of reflections is larger, the attenuation amount of high frequency components in the repeatedly reflected sound increases. Further, the multipliers 123 serve to further simulate a repeatedly reflected sound which progressively declines in amplitude.
The APF's 128-131 serve to further increase the density of dense output waveforms from the comb filters 125 to thereby simulate natural reverberation sound with a higher degree of high fidelity.
FIG. 3 schematically shows the interior construction of the APF's 128-131. The APF's are each mainly composed of a delay circuit 131, and adders 132 and 133. The delay circuit 131 generates through a reading output (reading location) E thereof an output signal with a predetermined amount of delay relative to an input signal thereto.
The proposed reflective reverberation-imparting device constructed as above is capable of imparting delay effects such as a reverberation effect to an input signal thereto to form a reverberation sound fairly close to natural sound.
However, output Waveforms formed by the comb filters 125 necessarily have delay characteristics inherent in the comb filters 125 due to the finite number of the comb filters 125 employed, even though the delay lines 121 are set to different delay times from each other so as to increase the density of the subsequent reflected sound or reverberation sound waveform S to be obtained, and the APF's 128-131 further increase the density of the sound waveform S.
FIG. 4 shows, by way of example, output timing of a signal from one of the delay lines 121. The time interval of generation of output pulses from the delay line 121, i.e. delay time difference, is always constant due to the constant delay time, though different delay times are set between the individual delay lines 121. Consequently, the resulting reproduced sound has a frequency characteristic dependent upon the delay characteristic inherent in the delay circuit 121, which sometimes gives the listener a feeling of difference from actual reverberation sound listened to in a hall or the like.